Description (Applicant Abstract): Our studies to date indicate that under 254 nm irradiation (Hg lamp) tryptophan residues and/or the retinyl protonated Schiff base chromophore (RETPSB) of bacteriorhodopsin (bR) generate one or more reactive intermediates which subsequently react with the RETPSB chromophore, leading to a complete loss of the visible absorbance of the protein. Results from our laser based studies have demonstrated that irradiation from a pulsed laser system operating at 532 nm leads to photodegradation of RETPSB chromophore, and this process appears to be mediated through protein-protein and/or protein-membrane interactions. These results are significant due to the fact that bR is an accepted model for the visual pigment rhodopsin, and that rhodopsin-mediated photodegradation (RH-MP) is believed to be in part responsible for age-related macular degeneration (AMD), a leading cause of vision impairment in the elderly. Therefore, by understanding the photochemistry of bR, it may be possible to better understand the cause of AMD, and ultimately to prevent it from occurring. To this end, the four principal goals of the proposed research are as follows: (1) To fully characterize the mechanism(s) responsible for the light-induced photodegradation of the model visual pigment bR which we have observed in our laboratory; (2) Identify the reactive intermediates through the use of competitive quenching studies and electron spin resonance experiments (ESR); (3) To test our proposed biphotonic mechanism for laser induced photodegradation; and (4) To gain new insights into apoprotein-chromophore interactions in visual and photosynthetic pigments.